Travels in Afghanistan

 

Dust devil near our compound in G. Dust devils are created when air near the surface becomes much warmer than the air above.  This creates an instability in which the warm air rises. With intense insolation, temperatures well above 100^oF up to 140^oF (40 to 60°C) are not uncommon on the desert floors during the summer months.  Dust devils typically occur in open areas where large quantities of air are heated.  An isolated area begins to rise due to its instability and soon much of the surrounding air rushes in and intensifies this upward movement.  The Coriolis effect gives the dust devil its characteristic spin (counterclockwise in the northern hemisphere).  The open, flat desert floor also minimizes frictional drag so the rush of hot air towards the rising vortex is maintained.  The abundance of dust enhances our view of the air movement.

This isolated dust devil nicely shows the cylindrical shape created by the ascending air.  Commonly a dust devil will begin as a large, slowly rotating mass of air.  There may even be multiple small dust devils within the mass, but as the rush of air towards the uplifting vortex increases, a single dust devil emerges.  The edges of this dust devil appear more heavily laden with debris due to the fact that the central region of the rising column is relatively clear of dust.

according to: http://www.gc.maricopa.edu/earthsci/imagearchive/dust_devils.htm